I think I've heard him correctly. "If you've never read it before, don't go for hard space opera." That is Peter F Hamilton's advice for people like me, who have never read science fiction and find its spaceships, hyperdrives and quantum fields utterly baffling. Hamilton is one of the most popular authors of "space operas" in Britain today, writing vast doorsteps of novels that combine fantastic speculation with incredibly detailed imagining of the lives we will lead some time after the 30th century. Is he saying his new book is not very accessible? "Um, to somebody who has never ever read it [science fiction] before I would suggest I'm not your best first choice," he says disarmingly. Then he relents slightly. "If you've enjoyed Battlestar Galactica, you should love my stuff," he says. And if you like whodunnits, he can recommend the Greg Mandel sci-fi/detective series that made his name.

"People are put off by the perception of science fiction," says Hamilton, "and it doesn't help if you've got references to quantum this and quantum that on the first page, and people think 'this isn't for me' and chuck it. I'm probably a pretty bad offender, given how far in the future some of my stuff is. But there is that little jump you have to make if you haven't read it before."

The number of people who love Doctor Who and Star Trek is, however, enormous. Hamilton, writing out of his shed in rural Rutland, is one of those authors who sells by the, er, shedload - 2m and rising - but rarely gets noticed beyond the sci-fi ghetto. His latest, The Temporal Void, is the middle book of his unfolding Void trilogy. I stupidly dived straight into it and found myself on another planet. It wasn't only the sentences - "he'd simply stealthed up and dropped into hyperdrive. Now he was sitting in transdimensional suspension 10 light years out from Sholapur" - but a profusion of terms like gaiafield, unisphere and u-shadow. More sensibly, I then went to the first in the trilogy, The Dreaming Void, and discovered that if you start from the beginning, Hamilton's storytelling is crystal clear. The sci-fi great Brian Aldiss has called space opera "widescreen baroque", and with a gigantic sweep of their keyboards, writers such as Hamilton create densely imagined future worlds riven with timeless human preoccupations - politics, religion, sex and money. Widescreen means big: writing while listening to Muse (Black Holes & Revelations, naturally), Pink Floyd and Mike Oldfield's Tubular Bells on his iPod, Hamilton churns out these huge tomes almost annually. His biggest trilogy, The Night's Dawn, weighed in at 1.2m words. The Void is more modest - at only 750,000.

The void in question is at the heart of the Commonwealth, our universe in the 34th century. Alien races have been discovered, but life is still mostly a story of human struggle. Humans are stratified into highers, who have abandoned their bodies altogether to pursue an enlightened, mostly bodyless existence, and others who have thwarted ageing with genetic manipulation. But the biggest conflict has been created by the void at the centre of the universe. Based on the dreams of a messianic figure, a new religion has sprung up theorising that paradise can be found in this void. A pilgrimage is launched, but other human and alien factions take the more rational view that journeying into the void will trigger a "devourment phase", in which the void will swallow the entire universe.

Hamilton believes a better term for sci-fi, or SF, is speculative fiction. "In no way is it ever predictive," he says. Here, however, he has form: his first novel, written in the late 80s, was set 40 years into the future after catastrophic global warming and a "credit crash". Nevertheless, he feels his earlier work is "creaking along now", such is the pace of technological change. "It's showing its age. My pride and joy at the time [of his first novel] was a cybofax. Because people had Filofaxes and personal computers were just coming in, I combined this into a book-sized thing with a colour screen and keyboard which you opened up. That was supposed to be set about 20 years from now! Hands up, I got caught out. It shows that near-term future is a lot harder than far-future."

Hamilton, 48, who is married to an accountant and has two young children, grew up in Rutland. "I love it dearly and it looks beautiful, but when you were a teenager in the 70s it was pretty dull," he says. "So when I found the sci-fi shelf in the library it was a way out. We'd just had the moon landings and we were building the shuttle which was supposed to be going up once a week. I guess my generation was a littl e bit more optimistic. It's all doom and gloom these days." He enjoyed all the sci-fi classics - Isaac Asimov, Robert A Heinlein, Arthur C Clarke - but only began writing in his 20s when he quit work in London and returned to the county to help look after his sick mother. He writes, he says, for himself, as a sci-fi fan. "If you're that 14-year-old stuck in a rut, it's great escapism, there are men with big guns and that kind of thing. But you could read it and see other stuff in there as well - religion and politics."

Within his enormous imagined world, Hamilton breathlessly weaves together dozens of subplots and characters, from feisty female detectives to nerdy physicists. While there is plenty of adolescent fun and geeky dialogue - "everyone's scared crapless" - he devises all too recognisable political squabbles and imaginatively shows how the internet, for instance, might develop. Hamilton reads as much contemporary popular science as he can, and likes to extrapolate from the latest theories, but also keeps it simple: "The hyperdrive is a black box with a button on that you press and it takes you where you want to go. Do not open the box and try to describe the circuitry inside. That's the basic rule in science fiction."

Sci-fi enjoyed a golden age in the 1930s, when another rule was established - no sex. Hamilton's liberal lashings of shagging - in the Void books it is mostly of the shy-messiah-with-three-genetically-enhanced-beauties variety - have proved controversial in the sci-fi community. "I've got a friend who writes a lot of chick lit. Compared to what goes into that genre we're absolute puritans, yet the reaction you get is 'Oh my God, it's a sex scene,'" he says. Sex is a tiny part of his books. "But you can't ignore it because that's part of what we are. We spend most of our lives in relationships or bringing up children which are a product of relationships."

A timeless human preoccupation of more importance in the Void trilogy is religion. "The whole point of science fiction is that you explore the effect of ideas on a society," says Hamilton. For centuries, religion's prime means of communication has been the written word. In the Void, the gaiafield, a kind of emotional internet through which people can directly share dreams and emotions, helps the emergence of a powerful new religion called the Living Dream. It is not inspired by one particular faith. "It's an amalgamation of religions. It's the idea of religions - we and only we can provide the answer and we are right to do so, " he says. "The answer that Living Dream are going to provide will screw the rest of the galaxy but they think they have an absolute right to follow that goal. What religion is that not?"

Is Hamilton an atheist? "I'm not quite an atheist but almost. There's a margin of error. There's a possibility. Not everything can be explained perfectly yet," he says. "I don't believe in supernatural entities taking on human form and telling us how to live our lives, but it's a very grey area."

It seems certain that Hamilton's latest offering from the Void will be another bestseller that evades the attentions of the literary establishment as effectively as any stealth ingrav ultradrive contraption. When a writer once dared to suggest to Auberon Waugh that the

Literary Review should have a sci-fi column, he replied that the magazine already employed a sci-fi editor: their job was to ensure no sci-fi ever darkened its doors. Does this kind of snobbery bother him? "It's irritating in the sense that we are shoved into a once-a-month roundup. If it's good or bad - I've been trashed many times by reviewers - or an interesting read, then it should be given a reasonable amount of exposure," he says. "It's this segregation thing - people complain that it's on the science fiction shelf at the back of the bookshop, but then that's where people know to go and get it. It helps us and it frustrates us at the same time."